Where does an official fuel economy figure come from?
From a car on a chassis dynamometer, a set of rollers in a climate-controlled room, driven through a scripted speed profile by a technician or a robot. The exhaust is collected and analysed, and fuel consumption is calculated from the carbon in it. No public road is involved. Every official mpg, L/100km or km/L figure in the world is produced this way.
The scripted speed profile is the test cycle, and it is the cycle that makes the difference between one country's figures and another's. A cycle that spends more time at low speed with gentle acceleration produces a better number than one that includes hard acceleration and sustained motorway speed. The car is the same; the script is not.
Laboratory testing exists because it is repeatable. Two cars tested a year apart on the same cycle can be compared fairly, which is the point of a rating. The price of that repeatability is that the conditions are kinder than real roads, a gap the real-world MPG guide looks at in detail.
What does the EPA rating measure?
The United States label carries three numbers: city, highway and combined, all in miles per US gallon. Since the 2008 model year the figures come from five test cycles, including one at high speed with hard acceleration, one with the air conditioning running in 95 degree heat, and one cold start at 20 degrees Fahrenheit. The combined figure weights city at 55% and highway at 45%.
The 2008 change matters when reading older figures. Before it, the EPA used two cycles from the 1970s and adjusted the results downward by a fixed percentage. The five-cycle method produces lower, more realistic numbers, so a 1998 rating of 30 mpg and a 2018 rating of 30 mpg do not describe equally efficient cars. The older car would score lower on today's test.
EPA figures are the most conservative of the major systems and typically the closest to what drivers achieve. Converting one to L/100km with the MPG to L/100km converter gives a metric figure that is still an EPA figure, tested on the EPA's cycles. It will usually look worse than the WLTP figure for the same car, and that is the test speaking, not the car.
What is WLTP and how does it differ from NEDC?
WLTP, the Worldwide Harmonised Light Vehicle Test Procedure, is the test behind every fuel economy figure for a new car in the European Union and the United Kingdom since September 2018. It replaced NEDC, the New European Driving Cycle, a test with roots in the 1970s that had become notorious for producing figures no driver could match.
The differences are concrete. NEDC lasted 20 minutes, covered 11 km, averaged 34 km/h and peaked at 120 km/h, with long stretches of steady speed and gentle acceleration. WLTP lasts 30 minutes, covers 23.25 km, averages 46.5 km/h and peaks at 131 km/h, with more dynamic driving and less idling. WLTP also tests the specific optional equipment fitted to the car, so a heavier trim on bigger wheels gets its own, worse figure.
The result is that WLTP numbers are typically 10 to 20% lower than NEDC numbers for the same car. A model quoted at 60 mpg under NEDC in 2016 might be quoted at 50 mpg under WLTP in 2019 with no mechanical change. Comparing across that boundary without knowing which test applied is a common source of confusion when reading older reviews.
UK figures are WLTP results expressed in miles per imperial gallon. EU figures are the same results in L/100km. They describe the same test and convert into each other exactly, using the UK mpg to L/100km page. What they do not convert into is an EPA figure, because the cycle is different.
What tests do Japan and India use?
Japan moved from its JC08 cycle to WLTC, the cycle within WLTP, for new models from 2018, and publishes results in km/L. JC08 was gentler still than NEDC, so Japanese catalogue figures from before 2018 look remarkably high: 30 km/L and above for small hybrids. WLTC figures for the same cars are lower, and Japan also publishes separate urban, suburban and highway values.
India certifies fuel economy through ARAI, the Automotive Research Association of India, on the Modified Indian Driving Cycle. Figures are published in km/L, written kmpl in advertising. The cycle is slow and gentle, reflecting Indian urban traffic, and owners commonly report real-world figures 15 to 25% below the certified number. The km/L to MPG converter turns a kmpl figure into the units used elsewhere.
China uses its own CLTC cycle, Australia and New Zealand use ADR 81/02 based on the old NEDC profile, and Canada uses a five-cycle method modelled on the EPA's but published in L/100km. Each produces figures that are internally consistent and externally hard to compare.
Can I compare a rating from one test with a rating from another?
Only roughly. The unit conversion is exact, but the test difference is not a fixed factor, because it depends on the car. A small petrol car with a stop-start system scores well on gentle cycles and loses more on demanding ones; a large diesel is more consistent across cycles. As a rough guide, WLTP figures run 10 to 20% above EPA for the same car, and older NEDC figures 20 to 35% above.
The honest approach is to compare like with like. Two cars with EPA ratings can be compared directly. Two cars with WLTP ratings can be compared directly. A car with an EPA rating and a car with a WLTP rating can be converted into the same unit, but the comparison should carry a mental margin of at least 10% in the EPA car's favour.
If both ratings are available for the same model, as they are for many cars sold in both the US and Europe, you can see the test difference directly. Convert the WLTP figure with the MPG US to UK converter to remove the gallon difference, and what remains is the effect of the cycle.
What about plug-in hybrids and electric cars?
Plug-in hybrid figures are the least meaningful of all. Under WLTP a PHEV is tested with a full battery and the electric miles are blended into the result, producing figures like 200 mpg or 1.2 L/100km that describe a short trip on a charged battery, not a tank of petrol. A PHEV driven without charging behaves like a heavy conventional hybrid and returns perhaps 35 to 45 mpg (US).
Electric cars are rated in kWh per 100 km or miles per kWh, and in the US in MPGe, a figure that converts electrical energy to the energy in a gallon of petrol at 33.7 kWh. MPGe is a useful efficiency comparison between electric cars and a poor guide to running cost, because electricity and petrol are priced very differently per unit of energy. This site does not convert MPGe because no litre figure sits behind it.